1. Field of the Invention
The present invention relates to reproduction of optical information recorded in optical recording media of high and low recording densities with an apparatus in a compatible way.
2. Description of Prior Art
Recently, DVDs (digital video disks) for providing high quality digital pictures and audio sound and optical disk drives or players for reproducing DVDs become very popular. The popularity of DVD players is ascribed to the high quality of DVDs, but it is also ascribed largely to the compatibility with previous media, that is, assurance to reproduce compact disks.
It seems easy at first to reproduce an optical disk of low recording density (for example, compact disk (CD)) in an optical disk drive for an optical disk of high recording density (for example, DVD). However, pit marks formed on an optical disk are designed to be optimum for a shape of a laser beam spot which illuminates the pit marks. Therefore, various problems have to be solved even for reproducing an optical disk of low recording density which deviates from the optimum conditions.
As a most typical example, so-called mirror phenomenon occurs when recording pits of low density are reproduced with a very small laser beam spot emitted from a pickup for high resolution. Digital visual and audio information is formed on an optical disk (for example, compact disk) as pits formed unevenly, while a film (for example, made of aluminum or gold) of the same reflectance is deposited on all of the plane thereof, irrespectively of the pits. Even if the reflectance is the same, information can be obtained by using diffraction at pit edges. Because a pit is formed to have width of about a half of a size of the laser beam spot, a part of the laser beam is diffracted at an edge of the pit so as not to enter a photodetector. However, when the pit is reproduced with a pickup of high resolution in correspondence to another optical disk (for example, DVD) of high density, because the laser beam spot is very small, almost all of the laser beam spot illuminates the inside of the pit. Thus, sufficient diffraction at the edge does not occur, and the reflected light increases. This is the so-called mirror phenomenon. It is assumed here that spherical aberration due to difference of the thickness of the substrates of two disks (for example, CD and DVD) is solved by an appropriate means.
For compatible reproduction of two types of optical disks, it is proposed in Japanese Patent laid open Publication 8-102079/1996 to enlarge the laser beam spot when information of low density is reproduced with a diaphragm. However, this needs an expensive optical component such as a nonlinear optical filter. It is also proposed in Japanese Patent laid open Publications 9-185833/1997 and 10-208276/1998 to change lenses of different numerical apertures. However, in order to change the lenses, a structure of pickup becomes complicated. It is further proposed to enlarge the laser beam spot size by limiting the aperture with a hologram or the like. However, this worsens efficiency of the laser beam due to diffraction loss at the hologram.